In designing automobile accessories, such as alternators, several factors must be considered. In the case of an alternator, there must be a compromise between acceptable speeds for the alternator and the desired alternator output. Tests show that the alternator output current drops drastically when the alternator speed falls below 3,000 rpm. On the other hand, the alternator output current rises only slightly when the alternator speed exceeds 10,000 rpm. Under ideal conditions, an alternator would operate at a continual speed change dependent on the required alternator output. Present alternator operation, however, relies on a standard fixed belt pulley ratio from the crankshaft of the engine to the alternator input pulley, thus alternator speed changes with engine speed. This ratio is determined by the required minimum alternator output current and the minimum engine idle speed. While this method may work well at lower engine speeds, higher engine speeds cause excessive alternator speeds; consequently, alternator design becomes very difficult. Predicted future trends show that alternator loads will increase dramatically and, in some instances, this is already a problem.
Possible solutions to this problem include (1) increasing the pulley ratio; (2) increasing the engine idle speed; (3) adding another alternator; and (4) developing a multi-speed alternator drive unit. All of these solutions have certain disadvantages affecting the vehicle's drivability and performance, however, a two-speed drive unit could be developed that would be cost effective and easily packaged. The present invention relates to a two-speed accessory drive unit to address this problem.